In looking around the Desiring God website, I came across a very helpful little resource. It is named simply Ask Pastor John. This is a place where John Piper receives and answers questions from all across the spectrum of the bible, theology, the church, ethics, etc.
Today's question is, "Is it possible to be angry and not sin?" A few days ago, someone asked, "What is God's glory?" Piper does his best to answer these questions in a short enough manner that they can be read or listened to in just a few minutes.
Does John Piper know everything? No. Is he well-versed in the scriptures? Yes. Does he have years of pastoral experience and wisdom? Yes. Therefore, I think it is worth it to listen to him answer some interesting and difficult questions.